Gear shift device for bicycles

ABSTRACT

A motorized bicycle gear shift device comprising a derailleur and an electric motor, associated directly with the derailleur, for controlling the derailleur and an electric transducer of absolute type, also associated with the derailleur for producing an electric signal indicative of the absolute position of the movable body of the derailleur.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Pat. Nos. 6,623,389 and7,048,659, which in turn claim priority from Italian Patent ApplicationNo. T099A001023, filed Nov. 23, 1999. U.S. Pat. Nos. 6,623,389 and7,048,659 are incorporated herein by reference as if fully set forth.

BACKGROUND

The present invention refers to a gear shift device for bicycles havingat least one derailleur, a first body destined to be attached to abicycle frame and a second body that can be displaced with respect tothe first body to cause the selective engagement of a bicycle chain on anumber of sprockets, to achieve the various gears of the device, thedevice comprising:

an electric actuator to drive the displacement of the second body of thederailleur,

electronic control means for controlling the electric actuator,

manually-operated activating means, connected to the electronic controlmeans, to cause activation of the electric actuator in order to selectthe required gear,

means for detecting the position of the second body of the derailleurand for indicating the position to the control means, so as to permitthe automatic disabling of the electric actuator once the requiredposition has been reached,

in which the electric actuator is directly associated with thederailleur.

A gear shift device of the kind outlined above is described andillustrated in U.S. Pat. No. 5,480,356, a patent held by the sameapplicant.

FIG. 1 of the attached drawings illustrates a racing bicycle, indicatedas a whole by reference number 1, comprising a frame 2 constructed, in awell-known manner, of tubular elements comprising stays 3 for supportingthe rear wheel 4. Reference number 5 indicates a fork for supporting afront wheel 6, associated with a handlebar 70, which is also made with atubular structure.

In its lower part, the frame 2 carries a crankset 7 of substantiallyconventional type for driving the rear wheel 4 by means of a gear shiftdevice made according to the present invention, indicated globally bythe number 8.

The gear shift device 8 essentially includes a rear assembly 9 and afront assembly 10 comprised, in a well-known manner, of a number ofsprockets 11 of various diameters, co-axial with the axle A of the rearwheel 4, and a number of crown wheels 12, also of various diameters,co-axial with the axle B of the bicycle's crankset 7.

The sprockets 11 and the crown wheels 12 can be engaged selectively by achain transmission forming a closed ring 13 to achieve the differentgear ratios available by means of the gear shift device 8, by activatinga rear derailleur 14 forming part of the rear assembly 9 and a frontderailleur 15 belonging to the front assembly 10.

FIG. 2 in the enclosed drawings illustrates the rear derailleur 14 madeaccording to the solution proposed in U.S. Pat. No. 5,480,356. The rearderailleur 14 includes a first body 16 that is designed to be attachedto the bicycle frame and a second body 17 connected to the first body 16by means of a parallelogram linkage comprising two arms 18 and 19, theends of which are articulated at 20, 21 and 22, 23 to the two bodies 16and 17. The second body 17 includes, in a well-known manner, a rockerarm 24 that carries the chain transmission wheels 25 and 26.

Reference number 27 indicates an electric actuator, in the form of anelectric motor combined with a reduction gear 29, that is directlyincorporated in the derailleur 14 to drive the displacement of thesecond body 17, and consequently also of the rocker arm 24, through thevarious engagement positions of the chain 3 with the sprockets 11.

FIG. 3 in the enclosed drawings illustrates the motor and reduction gearassembly 27 forming the object of the device described in U.S. Pat. No.5,480,356. In the Figure, the body of the motor and reduction gearassembly 27, indicated as 28, is shown on a larger scale and incross-section. The body 28 contains not only the motor 35, but also anepicycloid reduction gear 29 connected to the shaft coming from theelectric motor 35. The epicycloid reduction gear 29 drives the rotationof a screw 30. As shown in FIG. 2, the body 28 of the motor andreduction gear assembly is attached with an articulated coupling to thebody 16 of the derailleur around an axis 22, while the screw 30 engagesa nut screw 31, the body of which is mounted in an articulated manneraround an axis 21 on the body 17 of the derailleur. As a result, themotor and gear reduction assembly lies along a diagonal of theparallelogram linkage and the turning of the motor causes acorresponding rotation of the screw 30 by the epicycloid gearing 29, sothat the nut screw 31 moves along the screw, leading to an elongation orshortening of the distance between the axes 21 and 22 of theparallelogram linkage.

As illustrated in FIG. 3, the body 28 of the motor and reduction gearassembly also contains a device 32 comprising an encoder, including anoptical or magnetic sensor that co-operates in a well-known manner witha disc 34 carried by the screw 30.

The electric power to the motor and reduction gear assembly 27 isprovided by means of a battery 37 (FIG. 1) conveniently housed in one ofthe tubes of the bicycle frame 2 or, alternatively, in one side of thehandlebar 70, or inside the container of a microprocessor control unit40 (only partially visible in FIG. 1) that may be attached, forinstance, to the bicycle frame in the area of the crankset and is usedto control the electric motor 35 on the basis of signals originatingfrom two manually-operated control levers 43 and 44 (which could also bereplaced by two buttons) associated, in a well-known manner, with abrake lever 41 (FIG. 1). The microprocessor unit 40 is also connected tothe encoder 32, which detects the angular position of the screw 30, andconsequently of the rear derailleur, so as to stop the electric motorwhen a required transmission gear has been reached, the gear beingselected by manually operating the levers 43 and 44 (which are operatedto shift the chain into higher or lower gears, respectively). Theconnections between the aforementioned electric components are made, inthe case of the above-mentioned well-known solution, by means of wires(not shown in the drawings) conveniently positioned inside the tubes ofthe bicycle frame 2.

The above-mentioned earlier document describes the opportunity to use anencoder of any kind to provide the means for detecting the position ofthe second body of the derailleur. In the course of experience gained inthe past by the applicant, an incremental encoder was used. With thistype of transducer, however, any interruption in the electric powersupply to the encoder made it necessary to provide for a procedure forre-calibrating the encoder by making the derailleur move to the positiontaken as the zero reference by the encoder. It is also important to bearin mind that there is an increasingly-felt need to increase the autonomyof the bicycle with battery-powered systems and therefore to reduce theconsumption of electrical energy. For this purpose, control systems canbe used that enable energy to be delivered only for the amount of timeit takes to satisfy the requirement, after which the system returns to alow-consumption regime. It is also true that, even using an incrementaltransducer, strategies can be adopted to keep the position valuedetected memorized at logic level in the control unit. However, thissolution may not be sufficient to guarantee the reliability of thetransducer, because while it is not being powered the position of thesecond body of the derailleur may undergo small variations due to thevibrations to which the bicycle is liable while in motion.

SUMMARY

The purpose of the present invention is to achieve a gear shift deviceof the type outlined at the beginning of this description, in which themeans for detecting the position of the movable body of the gear gives areliable and accurate indication of the position, even in the event ofan interruption in the power supply, for instance.

With a view to achieving the purpose, the object of the invention is agear shift device having all the characteristics illustrated at thebeginning of this description and characterized, moreover, in that thedetection means are comprised of an absolute electric transducerassociated with the derailleur and designed to produce an electricsignal indicating the absolute position of the second body of thederailleur.

Thanks to the characteristic, the transducer is always capable ofproviding accurate information on the position of the second body of thederailleur, without requiring any re-calibration procedures after aninterruption in the power supply.

In a preferred embodiment, the transducer is a potentiometer comprisingat least one electrically-conducting track and a contact sliding on thetrack due to the effect of any movement of the second body of thederailleur with respect to the first body. In a first example of theembodiment, the potentiometer is of the rotating type, comprising atleast one circular track and one rotating movable contact engaged on thetrack. In a second example, the potentiometer is a sliding potentiometercomprising a cylinder carrying the aforementioned track and a rodcarrying the movable contact that slides inside the cylinder.

The absolute transducer can be associated directly with the actuator, orit can be placed between two parts of the derailleur that move inrelation to each other as a result of the movement of the second bodywith respect to the first body. For instance, in the case in which thederailleur has a parallelogram linkage that connects the first body tothe second body, a rotating potentiometer can be used in associationwith any of the articulations of the parallelogram linkage, or a slidingpotentiometer can be arranged between two parts of the parallelogramlinkage that move in relation to each other, or between the body of themotor and a nut screw that engages a screw driven by the motor.

In the case of the potentiometer being mounted on the shaft of theactuator, it is preferable for it to be placed downstream from areduction device so that the potentiometer performs less than one turnwhen the derailleur moves between its two end positions. Otherwise, areduction gear can be provided in the potentiometer itself or,alternatively, means for counting the turns of the shaft on which thepotentiometer is installed can be provided together with thepotentiometer.

The absolute transducer may not necessarily be a potentiometer; forinstance, it could be a resistive transducer or a transducer of opticalor magnetic type, such as a Hall-effect transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages of the invention will emerge fromthe description that follows with reference to the attached to drawings,which are provided purely by way of example and in no way restrict itsscope, in which:

FIG. 1 is a perspective view of a racing bicycle,

FIG. 2 is a perspective view of a rear derailleur made according to thewell-known technique,

FIG. 3 is a perspective exploded view of the motor and reduction gearassembly forming part of the derailleur in FIG. 2,

FIG. 4 is a partial cross-section of a front view of a first embodimentof a rear derailleur made according to the present invention,

FIG. 5 is a cross-section along the line V-V of FIG. 4,

FIG. 6 is a cross-section along the line VI-VI of FIG. 5,

FIG. 7 is a perspective view of a second embodiment of a rear derailleurmade according to the present invention,

FIG. 8 is a front view of a rear derailleur made according to a thirdembodiment of the present invention,

FIG. 9 is a cross-section on an exploded scale of a detail of FIG. 8,

FIG. 10 is a perspective view of a front derailleur of a bicycle madeaccording to a first embodiment of the present invention,

FIG. 11 is a partial cross-section of the front derailleur of FIG. 10,

FIG. 12 is a view corresponding to the one in FIG. 11 that illustratesthe front derailleur in a different operating condition, and

FIGS. 13 and 14 illustrate a variant of the front derailleur illustratedin FIGS. 11 and 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 4 the parts corresponding to the ones illustrated in FIG. 2 areindicated by the same reference number. The general arrangement of therear derailleur illustrated in FIG. 4 is substantially the same as theone illustrated in FIG. 2. Here again, the motor and reduction gearassembly has a body 28 articulated at 22 to the first body 16 of thederailleur and controls a screw 30 that engages a nut screw 31articulated at 21 to the second body 17 of the derailleur. In the caseof FIG. 4, however, the means for detecting the position of the secondbody 17 are comprised not of an encoder associated with the motor andgear reduction assembly 27, but of a transducer 50 mounted in line withthe articulation 21 of the parallelogram linkage. To be more specific,the transducer 50 is an absolute transducer, i.e. designed to produce anelectric output signal indicating the absolute position of the secondbody 17. In the embodiment of FIG. 4, the transducer 50 is comprised ofa rotating potentiometer capable of detecting the absolute angularposition of the second body 17 with respect to a pin 51 (see FIGS. 5 and6) which achieves the articulation of the body 17 on the arm 18 of theparallelogram articulated around the axis 21. The pin 51 rotates freelywith respect to the body 17 and is rigidly connected to the arm 18 ofthe parallelogram linkage, so a certain relative rotation of the pin 51around the body 17 unequivocally corresponds to a certain position ofthe body 17. As shown in FIGS. 5 and 6, the transducer 50 includes twocircular electric tracks 52, both supported by the body 17 lyingcoaxially to the axis 21, and a rotating contact 53, that moves with thepin 51 and has two points of contact 54 (see FIG. 5) engaged in slidingcontact respectively with the two tracks 52. The two tracks 52 areelectrically connected by means of a cable 55 to the power supply andthe movable contact 53 is used to close the circuit between the twotracks, and any variation in the position of the movable contact 53gives rise to a variation in the electrical resistance which can bemeasured, in a well-known manner, in order to produce a signalindicating the absolute position of the movable body 17. The arrangementshown in FIG. 6 would be an example of an arrangement that would bepossible for use as a magnetic (e.g. Hall-effect, presently preferred)or optical transducer 50 as well. In such arrangements, the rotation ofthe contact 53 and pin 51 is either magnetically or optically detected.

FIG. 7 illustrates a second embodiment that only differs from the oneillustrated in FIG. 4 in the fact that the transducer 50 is arranged notin line with the articulation 21, but with another articulation of theparallelogram linkage instead (i.e. articulation 23). Otherwise, thestructure of the derailleur and of the transducer 50 is identical to theone described with reference to FIGS. 4-6.

FIG. 8 illustrates a further embodiment in which an absolute transducer56 is used, comprised of a sliding cylinder-type potentiometer. Theconstruction of the potentiometer is schematically illustrated in FIG.9. The potentiometer includes a cylinder 57 that contains two electrictracks 58 extending face-to-face, powered by means of a cable 59. Insidethe cylinder 57 there is a sliding rod 60 with an element 61 inside thecylinder, rather like a piston, which acts as a movable contact designedto close the circuit by connecting the two tracks 58 together, so that,here again, the electrical resistance of the assembly depends on theposition of the rod 60.

As illustrated in FIG. 8, the transducer 56, like the motor and gearreduction assembly 27, also lies substantially along a diagonal of theparallelogram linkage (on an outer side of the parallelogram, where thetransducer 56 does not interfere with the motor). The cylinder 57 isarticulated at 62 to the fixed body 16, while the rod 60 is articulatedat 63 to an appendix 18 a on the arm 18 of the parallelogram linkage(similar to one generally used in conventional derailleurs for couplingthe flexible control cable).

FIGS. 10-12 illustrate a further example of implementation of theinvention applied to the case of a front derailleur. Here again, thefront derailleur is comprised, in a well-known manner, of a first body16, designed to be attached in some known manner to the bicycle frame,and a second body 17 shaped according to the conventional method in theform of a fork, to control the selective engagement of the chain on thecrown wheels 12 associated with the crankset. The body 17 is alsoconnected to the fixed body 16, again in a well-known manner, by meansof a parallelogram linkage comprising two arms 18 and 19, articulated at20, 21 and 22, 23 respectively to the fixed body 16 and the movable body17. Moreover, the arm 18 extends, in the case of the present invention,into a lever 80, whose movement is controlled by a motor and reductiongear assembly 27 of the kind described previously, mounted on the fixedbody 16 so as to oscillate around an axis 90 parallel to the axes 20-23of the parallelogram linkage. The motor and reduction gear assembly 27causes the rotation of a screw 30 that engages a nut screw 31articulated at 81 to the lever 80. FIGS. 11 and 12 illustrate thefork-shaped movable body 17 in its two end positions.

As in the case of the rear derailleur, the position of the movable body17 is detected, here again, by means of an absolute transducer 50. Inthe example of FIGS. 11 and 12, the transducer 50 is a rotatingpotentiometer, associated with the articulation 20, and comprising arotating contact 53 connected to the arm 18 and to the lever 80, slidingover an arched track 52 on the fixed body 16.

Finally, FIGS. 13 and 14 illustrate a variant that is substantiallyidentical to the one illustrated in FIGS. 11 and 12, except for the factthat in this case a transducer 56 comprised of a sliding potentiometeris used, comprising a rod 60 engaged in sliding contact inside a guideelement 57. The construction of the inside of the transducer 56 can bemuch the same as the one illustrated with reference to FIG. 9, in thesense that the rod 60 can carry a movable contact designed to connecttwo opposite tracks placed on the inside surface of the guide element57.

It appears evident from the previous description that the principlelying at the basis of the present invention is that of achieving amotorized gear shift device using a motor directly associated with thederailleur and a transducer for detecting the position of the movablebody controlled by the motor, which is a transducer of absolute type,i.e. designed to produce an output signal indicative of the absoluteposition of the movable body. Although the achievement of the transducerin the form of a potentiometer is preferred, any other type of absolutetransducer could also be used.

For instance, optical or even magnetic (e.g. Hall-effect) types ofabsolute transducers 50,56 suitable for producing an output signalindicative of the absolute position of the element detected are alsowell-known. These alternative transducer types may be configured in thesame manner that the transducers 50,56 are configured. Further,resistive transducers 50,56 can be used. In a resistive transducer, theresistance changes in response to the value of the physical quantitybeing measured. A potentiometer is one kind of resistive transducer.

It is equally evident from the preceding description that, thanks to theuse of an absolute transducer, the device made according to thisinvention is always capable of providing a reliable and accurateindication of the position of the element being displaced, regardless ofany interruptions in the power supply, or any bumping or vibrations thatthe device may suffer when the bicycle is in motion.

It is also clear that the positioning of the absolute transducer canvary considerably with respect to the one described and illustrated hereby way of example. As we have seen in the case of a derailleur with aparallelogram linkage, the transducer can be placed in line with any ofthe articulations of the parallelogram linkage, or it can be placedbetween any two parts of the derailleur that move in relation to eachother as a result of any displacement of the movable body of thederailleur. Alternatively, the transducer can be associated directlywith the controlling motor and reduction gear assembly. Obviously, thesignal from the transducer is more accurate when the latter is mountedas close as possible to the element it monitors, so that its signal isnot influenced by any drive elements or by the corresponding slack.

Finally, as concerns the electric control motor, this can obviously bemade in any well-known manner. For instance, the motor can be a dc motorwith brushing contacts, an ultrasonic motor, a stepping motor, or abrushless motor.

The electric connections between the component parts of the device madeaccording to this invention can also be achieved with a wireless type oftechnology, using transmitter and receiver devices associated with thevarious components.

Of course, without prejudice to the principle of the invention, theconstruction details and types of implementation can vary considerablywith respect to the version described and illustrated here merely by wayof example, without departing from the context of the present invention.

1. A gear shift device for bicycles having at least one derailleur, afirst body adapted to be attached to a bicycle frame, and a second bodythat can be displaced with respect to the first body to cause theselective engagement of a bicycle chain on a number of sprockets, toachieve the various gear ratios of the device, the device comprising: anelectric actuator for displacing the second body relative to the firstbody; and an absolute transducer that operates independently from theactuator and is mounted on the device for detecting an absolute positionof the second body.
 2. The device according to claim 1 wherein thetransducer is a resistive transducer.
 3. The device according to claim 1wherein the transducer is a rotary potentiometer.
 4. The deviceaccording to claim 1 wherein the transducer is a linear potentiometer.5. The device according to claim 1 wherein the transducer is a magnetictransducer.
 6. The device according to claim 1 wherein the transducer isa rotary magnetic transducer.
 7. The device according to claim 1 whereinthe transducer is a linear magnetic transducer.
 8. The device accordingto claim 1 wherein the transducer is an optical transducer.
 9. Thedevice according to claim 1 wherein the transducer is a linear opticaltransducer.
 10. The device according to claim 1 wherein the transduceris a rotary optical transducer.
 11. The device according to claim 1wherein the transducer is a Hall-effect transducer.
 12. The device ofclaim 1, wherein the absolute transducer is separated from the electricactuator by a screw that is driven by the actuator.
 13. The device ofclaim 12, wherein when the actuator drives the screw, a distance betweenthe absolute transducer and the actuator changes.
 14. A gear shiftdevice for a bicycle having a transmission chain, a first body forattachment to a bicycle frame, and a second body, pivotably associatedto the first body, which selectively engages the transmission chain, thedevice comprising: an electric actuator mounted on the device thatdisplaces the second body relative to the first body; and an absolutetransducer separate and independent from the actuator and mounted on thedevice that detects an absolute position of the second body.
 15. Thedevice according to claim 14 wherein the transducer is a potentiometer.16. The device according to claim 14 wherein the transducer is amagnetic transducer.
 17. The device according to claim 16 wherein themagnetic transducer is a linear magnetic transducer.
 18. The deviceaccording to claim 14 wherein the transducer is an optical transducer.19. The device of claim 18 wherein the optical transducer is a rotaryoptical transducer.
 20. The device of claim 14 wherein the transducer isa Hall-effect transducer.
 21. A method for positioning and determiningthe position of a transmission chain on a bicycle having a frame and aplurality of sprockets which are selectively engaged by the transmissionchain, the method comprising: attaching a first body on the bicycleframe in proximity to the plurality of sprockets; pivotably associatinga second body to the first body for selectively positioning thetransmission chain on one of the plurality of sprockets; attaching anelectric actuator to the device which positions the second body relativeto the first body for positioning the transmission chain; attaching anabsolute transducer that operates independently from the electricactuator to the device for detecting an absolute position of the secondbody.
 22. The method of claim 21 wherein the transducer comprises apotentiometer.
 23. The method of claim 21 wherein the transducercomprises an optical transducer.
 24. The method of claim 21 wherein thetransducer comprises a magnetic transducer.
 25. The method of claim 21wherein the transducer is a Hall-effect transducer.
 26. A gear shiftdevice for bicycles having at least one derailleur, a first bodyattachable to a bicycle frame, and a second body that can be displacedwith respect to the first body, which in turn causes engagement of abicycle chain from one toothed crown to a toothed crown, the devicecomprising: an electric actuator that displaces the second body relativeto the first body when it receives a gear shifting request, saiddisplacement moving the chain from the one toothed crown to the secondtoothed crown; and a transducer separate and independent from theelectric actuator that detects at least two positions of the second bodyrelative to a position of the first body, wherein when one of saidpositions corresponds to a position where the chain is engaged with thesecond toothed crown, the transducer outputs a signal that selectivelydisables the electric actuator.
 27. The device of claim 26 wherein thetransducer is an absolute transducer that detects the positions of thesecond body relative to the first body.
 28. A gear shift device forbicycles having at least one derailleur, a first body attachable to abicycle frame, and a second body that can be displaced with respect tothe first body, which in turn causes engagement of a bicycle chain fromone toothed crown to a toothed crown, the device comprising: an electricactuator that displaces the second body relative to the first body whenit receives a gear shifting request, said displacement moving the chainfrom the one toothed crown to the second toothed crown; and a transducerthat operates independently from the electric actuator and detectsmultiple positions of one of the first or second bodies relative to theother of the first or second bodies, wherein when one of said positionscorresponds to a position where the chain is engaged with the secondtoothed crown, the transducer outputs a signal that selectively disablesthe electric actuator.
 29. The device of claim 28 wherein the transduceris an absolute transducer.
 30. A gear shift device for bicycles havingat least one derailleur, a first body attachable to a bicycle frame, anda second body that can be displaced with respect to the first body,which in turn causes engagement of a bicycle chain from one toothedcrown to a toothed crown, the device comprising: an electric actuatorthat displaces the second body relative to the first body when itreceives a gear shifting request, said displacement moving the chainfrom the one toothed crown to the second toothed crown; and an absolutetransducer that operates independently from the electric actuator anddetects at least two positions of the second body relative to a positionof the first body.
 31. A gear shift device for bicycles having at leastone derailleur, a first body adapted to be attached to a bicycle frame,and a second body that can be displaced with respect to the first bodyto cause the selective engagement of a bicycle chain on a number ofsprockets, to achieve the various gears of the device, the devicecomprising: an electric actuator for displacing the second body relativeto the first body; and a transducer that operates independently and isseparate from the electric actuator and is mounted on the device fordetecting a position of the second body relative to a position of thefirst body, the transducer outputting a signal that selectively disablesthe electric actuator.
 32. A gear shift device for a bicycle having atransmission chain, a first body for attachment to a bicycle frame, anda second body, pivotably associated to the first body, which selectivelyengages the transmission chain, the device comprising: an electricactuator mounted on the device that displaces the second body relativeto the first body; and a transducer that operates independently from theelectric actuator and is mounted on the device that detects a relativeposition of the second body with respect to the first body, wherein thetransducer outputs a signal that selectively disables the electricactuator.
 33. The device of claim 32, wherein the absolute transducer isseparated from the electric actuator by a screw that is driven by theactuator.
 34. The device of claim 33, wherein when the actuator drivesthe screw, a distance between the absolute transducer and the actuatorchanges.
 35. A method for positioning and determining the position of atransmission chain on a bicycle having a frame and a plurality ofsprockets which are selectively engaged by the transmission chain, themethod comprising: attaching a first body on the bicycle frame inproximity to the plurality of sprockets; pivotably associating a secondbody to the first body for selectively positioning the transmissionchain on one of the plurality of sprockets; attaching an electricactuator to the device which positions the second body relative to thefirst body for positioning the transmission chain; attaching atransducer that operates independently from the electric actuator to thedevice for detecting a position of the second body relative to aposition of the first body, wherein the transducer outputs a signal thatselectively disables the electric actuator.
 36. A gear shift device forbicycles having at least one derailleur, a first body adapted to beattached to a bicycle frame, and a second body that can be displacedwith respect to the first body to cause the selective engagement of abicycle chain on a number of sprockets, to achieve the various gears ofthe device, the device comprising: an electric actuator for displacingthe second body relative to the first body; and an absolute transducerthat operates independently from the electric actuator and is mounted onthe device for detecting an absolute position of the second body, thetransducer comprising two electric tracks and a contact with at leastone point engaged in sliding contact respectively with the two tracks,wherein variation in the position of the contact gives rise to avariation in electrical resistance and produces a corresponding signalindicating the absolute position of the second body.
 37. A gear shiftdevice for bicycles having at least one derailleur, a first body adaptedto be attached to a bicycle frame, and a second body that can bedisplaced with respect to the first body to cause the selectiveengagement of a bicycle chain on a number of sprockets, to achieve thevarious gear ratios of the device, the device comprising: an electricactuator for displacing the second body relative to the first body; andan absolute transducer separate and independent from the actuator, saidtransducer mounted on the device for detecting an absolute position ofthe second body.